Alpha-Fetoprotein Blood Test
What is the alpha-fetoprotein (AFP) blood test?
The most abundant plasma protein found in the human fetus is alpha-fetoprotein (AFP). AFP is a protein normally made by the immature liver cells in the fetus. Healthy, nonpregnant adults have very low levels of AFP in the bloodstream (undetectable to about 10 ng/ml). At birth, infants have relatively high levels of AFP in the blood, which fall to normal low adult levels by the first year of life.
Pregnant women carrying babies with neural tube defects may have high levels of AFP in both the bloodstream, urine, and in the amniotic fluid. A neural tube defect is an abnormal fetal brain or spinal cord that is caused by folic acid deficiency during pregnancy. Examples of these birth defects include spina bifida and anencephaly. Women carrying twins or other multiples may also have elevated levels of AFP due to the presence of multiple fetuses producing AFP. Lower than normal AFP levels in pregnant women are sometimes seen in pregnant women carrying babies with genetic conditions such as Down syndrome.
It is important to note that abnormal levels of AFP in pregnancy do not necessarily mean there is a problem with the baby. If AFP levels are not normal, it means that more tests should be done to determine the cause. The American Pregnancy Association says that all pregnant women should be offered an AFP test sometime between the 15th and 20th week of pregnancy. AFP is typically part of the triple screen or quad screen test (along with human chorionic gonadotropin or HCG, estriol, and inhibin A) used in the second trimester of pregnancy to determine whether additional testing is necessary.
Alpha-fetoprotein is also produced by certain cancers, such as liver or testicular cancer, and is sometimes measured as a tumor marker (see below).
The AFP tumor marker test can be performed on a blood sample, urine sample, or sample of amniotic fluid.. Other names for the test include total AFP, MSAFP (maternal serum AFP), and alpha-fetoprotein-L3 percent (%)
In which situations are high blood (serum) levels of AFP used as a tumor marker?
In nonpregnant adults, high blood levels (over 500 nanograms/milliliter [or ng/ml]) of AFP are seen in only a few situations, such as
- hepatocellular carcinoma (HCC), a primary cancer of the liver;
- germ cell tumors (a type of cancer of the testes and ovaries, such as embryonal carcinoma and yolk sac tumors); and